This invention relates to a biaxial loading apparatus and more particularly to a self-aligning biaxial load fixture for applying orthogonal axial loads to a test specimen while maintaining a constant specimen centroid during the loading operation.
Recent developments in carbon-carbon composites, ceramics and alloys have substantially advanced the state of the art and have resulted in the development of interest in such materials for use in structural applications. Currently, there is no way to test these materials in a multi-axial stress state without imparting some undesirable stresses to the materials being tested, due to alignment problems in the apparatus loading the test specimen.
Considerable effort has been made in an attempt to satisfy a long-felt need for determining properties of brittle materials such as carbon-carbon composites, ceramics and alloys. Most efforts have focused on compressive, torsional and tensile testing. A need exists for expanding capabilities to include biaxial testing, that is, axial compressive and/or tensile testing along orthogonal axes. Because brittle materials are extremely sensitive to misalignment during testing, it has not been possible to test these types of materials in orthogonal axial directions without introducing unacceptable moments in the test specimen. These types of tests are imperative, however, for developing design guidelines for structural applications of brittle materials. Accordingly, a means is needed that will maintain alignment of a brittle test specimen continually during a test involving biaxial loading.
Biaxial load testing machines are known in the art which apply a monoaxial load to a test piece from orthogonal directions. U.S. Pat. No. 3,797,303 describes such a device which applies monoaxial compression load to a test specimen. In this invention, the device provides two orthogonal compression assemblies carried independently on guides. However, with this type of device some amount of undesirable moments to the test specimen can occur during compression loading, resulting in inaccurate load bearing data of the specimen.
Also, a triaxial compression test apparatus is shown in U.S. Pat. No. 4,615,221 which tests cylindrically-shaped samples by imparting a compressive force along the longitudinal axis of the sample. This compressive force then creates a second force along the axial axis of the cylindrical sample through the use of hydraulic fluid surrounding the object. However, the application of this device is limited to compressive testing of cylindrical samples.
U.S. Pat. No. 4,686,860 describes a grip system for transmitting an uniaxial load to a ceramic specimen without introducing bending stresses into the specimen. In this device a multiplicity of hydraulic piston assemblies are equally spaced in a hydraulic housing assembly on a circle about the centerline of the tensile specimen. Use of a hydraulic fluid as a distribution medium to divide the applied uniaxial load into a multiplicity of equal parts counteracted by the circular array of miniature pistons is a key feature of this invention. However, frequently it is desired to apply a multiaxial load to a test specimen, including both tensile and/or compression testing.
A hydrostatic self-aligning axial/torsional mechanism is described in U.S. Pat. No. 4,928,532 for testing specimens without introducing bending moments induced by other testing means. The test specimen can be tested by this invention for uniaxial strength, torsional strength, or a combination of the two. However, this invention does not test for compressive and/or tensile axial strength from orthogonal directions and accordingly does not consider the inducement of bending moments in the test specimen during the loading operation. It should be noted that the term "biaxial" as used in U.S. Pat. No. 4,928,532 refers to a combined axial and torsional loading, whereas in the present invention the term "biaxial" is understood to mean axial loading along orthogonal axes.
Accordingly, it is an object of this invention to provide for a means to test the compressive and/or tensile axial strength of a specimen in orthogonal directions.
It is another object of this invention to provide a means for maintaining the orthogonal alignment of the test specimen during testing.
Another object of this invention is to provide a means for applying orthogonal axial stress to a specimen without inducing significant moments in the specimen.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.